One volt is defined as the electric potential between two points of a
conducting wire when an
electric current of one
ampere dissipates one
watt of
power between those points. It can be expressed in terms of SI base units (
m,
kg,
s, and
A) as : \text{V} = \frac{\text{power}}{\text{electric current}} = \frac{\text{W}}{\text{A}} = \frac{\text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}}{\text{A}} = \text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}{\cdot}{\text{A}^{-1}}. Equivalently, it is the potential difference between two points that will impart one
joule of
energy per
coulomb of charge that passes through it. It can be expressed in terms of SI base units (
m,
kg,
s, and
A) as : \text{V} = \frac{\text{potential energy}}{\text{charge}} = \frac{\text{J}}{\text{C}} = \frac{\text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-2}}{\text{A}{\cdot}\text{s}} = \text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}{\cdot}{\text{A}^{-1}}. It can also be expressed as amperes times
ohms (current times resistance,
Ohm's law),
webers per second (magnetic flux per time), watts per ampere (power per current), or joules per coulomb (energy per charge), which is also equivalent to
electronvolts per
elementary charge: : \text{V} = \text{A}{\cdot}\Omega = \frac{\text{Wb}}{\text{s}} = \frac{\text{W}}{\text{A}} = \frac{\text{J}}{\text{C}} = \frac{\text{eV}}{e}.
Josephson junction definition Historically the "
conventional" volt,
V90, defined in 1987 by the 18th
General Conference on Weights and Measures and in use from 1990 to 2019, was implemented using the
Josephson effect for exact frequency-to-voltage conversion, combined with the
caesium frequency standard. Though the Josephson effect is still used to realize a volt, the constant used has changed slightly. For the
Josephson constant,
KJ = 2
e/
h (where
e is the
elementary charge and
h is the
Planck constant), a "conventional" value
KJ-90 = was used for the purpose of defining the volt. As a consequence of the
2019 revision of the SI, as of 2019 the Josephson constant has an exact value of = , which replaced the conventional value
KJ-90. This standard is typically realized using a series-connected array of several thousand or tens of thousands of
junctions, excited by microwave signals between 10 and 80 GHz (depending on the array design). Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc., and no correction terms are required in a practical implementation. == Water-flow analogy ==